ATP is a key mediator of central and peripheral chemosensory transduction

Astrocytic contribution to glutamate-related central respiratory chemoreception in vertebrates

Central respiratory chemoreceptors play a key role in the respiratory homeostasis by sensing CO₂ and H⁺ in brain and activating the respiratory neural network. This ability of specific brain regions to respond to acidosis and hypercapnia is based on neuronal and glial mechanisms. Several decades ago, glutamatergic transmission was proposed to be involved as a main mechanism in central chemoreception. However, a complete identification of mechanism has been elusive. At the rostral medulla, chemosensitive neurons of the retrotrapezoid nucleus (RTN) are glutamatergic and they are stimulated by ATP released by RTN astrocytes in response to hypercapnia. In addition, recent findings show that caudal medullary astrocytes in brainstem can also contribute as CO₂ and H⁺ sensors that release D-serine and glutamate, both gliotransmitters able to activate the respiratory neural network. In this review, we describe the mammalian astrocytic glutamatergic contribution to the central respiratory chemoreception trying to trace in vertebrates the emergence of several components involved in this process.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Angiotensin 1-7 in the rostro-ventrolateral medulla increases blood pressure and splanchnic sympathetic nerve activity in anesthetized rats

Angiotensin 1-7 (ANG-(1-7)), a derivative of angiotensin I or II, is involved in the propagation of sympathetic output to the heart and vasculature, and the receptor for ANG-(1-7), the Mas receptor, is expressed on astrocytes in the rostral ventrolateral medulla (RVLM). We recorded blood pressure (BP) and splanchnic sympathetic nerve activity (SSNA) before and after focal injection of ANG-(1-7) into the RVLM of rats. Unilateral injection of ANG-(1-7) into the RVLM, acting through the Mas receptor, increased SSNA and BP, and glutamate receptor antagonists, CNQX and D-AP5, partially reduced the ANG-(1-7) effect. ATP is often co-released with glutamate, and blocking ATP with PPADS also reduced the pressor response to microinjection of ANG-(1-7) within the RVLM. The effects of ANG-(1-7) were blocked by the MAS receptor antagonist, A-779 (which had no consistent effect on blood pressure or sympathetic nerve activity when injected on its own). We conclude that astrocytes in the RVLM participate in central, angiotensin-dependent regulation of blood pressure and sympathetic nerve activity, and the Mas receptor, when activated by ANG-(1-7), elicits the release of the gliotransmitters, glutamate and ATP. These gliotransmitters then cause an increase in sympathetic nerve activity and blood pressure by interacting with AMPA/kainate and P2X receptors in the RVLM.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

Independent purinergic mechanisms of central and peripheral chemoreception in the rostral ventrolateral medulla

The rostral ventrolateral medulla oblongata (RVLM) contains two functionally distinct types of neurons that control and orchestrate cardiovascular and respiratory responses to hypoxia and hypercapnia. One group is composed of the central chemoreceptor neurons of the retrotrapezoid nucleus, which provides a CO₂/H(+) -dependent drive to breathe and serves as an integration centre and a point of convergence of chemosensory information from other central and peripheral sites, including the carotid bodies. The second cluster of RVLM cells forms a population of neurons belonging to the C1 catecholaminergic group that controls sympathetic vasomotor tone in resting conditions and in conditions of hypoxia and hypercapnia. Recent evidence suggests that ATP-mediated purinergic signalling at the level of the RVLM co-ordinates cardiovascular and respiratory responses triggered by hypoxia and hypercapnia by activating retrotrapezoid nucleus and C1 neurons, respectively. The role of ATP-mediated signalling in the RVLM mechanisms of cardiovascular and respiratory activities is the main subject of this short review.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

Effects of intermittent hypoxia on leptin signalling in the carotid body

Glomus cells in the carotid body are responsible for detecting changes in the partial pressure of blood oxygen (PO₂). These glomus cells have recently been found to express leptin receptors and are activated by intermittent hypoxia (IH) and systemic leptin injections, although the function of leptin within the carotid body remains unknown. The present study was done to investigate whether IH activates leptin signalling pathways within leptin-expressing carotid body glomus cells. Rats were subjected to IH (120-s normoxia, 80-s hypoxia for 8 h) or normoxia (8 h). Exposure to IH increased plasma leptin levels almost sixfold compared to normoxic controls. Additionally, IH was found to increase leptin, ERK1/2 and Fra-1/2 immunoreactivity within glomus cells. Systemic leptin injections evoked similar effects on leptin, ERK1/2 and Fra-1/2 immunoreactivity within the glomus cells. Furthermore, using Western blot analysis, IH was found to increase protein expression of leptin, the short form of the leptin receptor (Ob-R₁₀₀ kDa) and suppressor of cytokine signalling 3. On the other hand, IH induced a decrease in long form of leptin receptors (Ob-Rb) protein expression. Taken together, these data suggest that the increased levels of leptin within the circulation and those within the glomus cells induced by IH may alter carotid bodies chemosensitivity to hypoxic stimuli.

Astrocytes as brain interoceptors

Astrocytes form a vascular-neuronal interface and provide CNS neural networks with essential structural and metabolic support. They embrace all penetrating arterioles and capillaries, enwrap multiple neuronal somata, thousands of individual synapses, and upon activation release gliotransmitters (ATP, glutamate and D-serine) capable of modulating neuronal activity. The aim of this brief report is to review recent data implicating astrocytes in the brain mechanisms responsible for the detection of different sensory modalities and transmitting sensory information to the relevant neural networks controlling vital behaviours. The concept of astrocytes as brain interoceptors is strongly supported by our recent data obtained from studies of the central nervous mechanisms underlying the chemosensory control of breathing. At the level of the medulla oblongata, astrocytes indeed act as functional central respiratory chemoreceptors, sensing changes in the arterial blood and brain levels of /pH and then imparting these changes on the activity of the respiratory network to induce adaptive changes in lung ventilation.

Ventilatory and carotid body chemoreceptor responses to purinergic P2X receptor antagonists in newborn rats

Adenosine triphosphate, acting through purinergic P2X receptors, has been shown to stimulate ventilation and increase carotid body chemoreceptor activity in adult rats. However, its role during postnatal development of the ventilatory response to hypoxia is yet unknown. Using whole body plethysmography, we measured ventilation in normoxia and in moderate hypoxia (12% fraction of inspired O₂, 20 min) before and after intraperitoneal injection of suramin (P2X₂ and P2X₃ receptor antagonist, 40 mg/kg) in 4-, 7-, 12-, and 21-day-old rats. Suramin reduced baseline breathing (∼20%) and the response to hypoxia (∼30%) in all rats, with a relatively constant effect across ages. We then tested the effect of the specific P2X₃ antagonist, A-317491 (150 mg/kg), in rats aged 4, 7, and 21 days. As with suramin, A-317491 reduced baseline ventilation (∼55%) and the hypoxic response (∼40%) at all ages studied. Single-unit carotid body chemoreceptor activity was recorded in vitro in 4-, 7-, and 21-day-old rats. Suramin (100 μM) and A-317491 (10 μM) significantly depressed the sinus nerve chemosensory discharge rate (∼80%) in normoxia (Po₂ ∼150 Torr) and hypoxia (Po₂ ∼60 Torr), and this decrease was constant across ages. We conclude that, in newborn rats, P2X purinergic receptors are involved in the regulation of breathing under basal and hypoxic condition, and P2X₃-containing receptors play a major role in carotid body function. However, these effects are not age dependent within the age range studied.

ATP, glia and central respiratory control

An increase in PCO(2) in the arterial blood triggers immediate release of ATP from the ventral chemosensory site(s) on the surface of the medulla oblongata. Systemic hypoxia in anesthetized rats was also associated with increased ATP release on the ventral medullary surface. During both hypoxia and hypercapnia, ATP and possibly other gliotransmitters released in the ventral medulla seemed to enhance cardiorespiratory responses to these stressors, and some of this ATP was proposed to be derived from astrocytes. Astrocytes also play a vital role controlling local blood flow. Astrocytes are activated by neurotransmitter release - especially glutamate and ATP. The astrocytic activation is manifest as a rise in intracellular Ca(2+) that is closely coupled to the metabolic activity of neurons in the active area. The activation of astrocytes spreads as a wave from astrocyte to astrocyte and causes release of ATP, adenosine, and other gliotransmitters that may alter neuronal function in the region of astrocytic activation. In addition, ATP, adenosine and other vasoactive substances, when released at the endfeet of astrocytes, interact with vascular receptors that may either dilate or constrict the vessels in the region closely adjacent to the site of neuronal activity. Thus, astrocytes seem to integrate neuronal metabolic needs by responding to the level of neuronal activity to regulate local blood flow and cardiorespiratory responses to hypoxia and hypercapnia to match substrate need (oxygen and glucose) with substrate availability and with the removal of CO(2). In so doing, astrocytes assume a larger role in information processing and in the regulation of neuronal activity and homeostasis of the entire organism than has been ascribed to them in the past.

Retrotrapezoid nucleus and parafacial respiratory group

The rat retrotrapezoid nucleus (RTN) contains about 2000 Phox2b-expressing glutamatergic neurons (ccRTN neurons; 800 in mice) with a well-understood developmental lineage. ccRTN neuron development fails in mice carrying a Phox2b mutation commonly present in the congenital central hypoventilation syndrome. In adulthood, ccRTN neurons regulate the breathing rate and intensity, and may regulate active expiration along with other neighboring respiratory neurons. Prenatally, ccRTN neurons form an autonomous oscillator (embryonic parafacial group, e-pF) that activates and possibly paces inspiration. The pacemaker properties of the ccRTN neurons probably vanish after birth to be replaced by synaptic drives. The neonatal parafacial respiratory group (pfRG) may represent a transitional phase during which ccRTN neurons lose their group pacemaker properties. ccRTN neurons are activated by acidification via an intrinsic mechanism or via ATP released by glia. In summary, throughout life, ccRTN neurons seem to be a critical hub for the regulation of CO(2) via breathing.

Central respiratory chemoreception

By definition central respiratory chemoreceptors (CRCs) are cells that are sensitive to changes in brain PCO(2) or pH and contribute to the stimulation of breathing elicited by hypercapnia or metabolic acidosis. CO(2) most likely works by lowering pH. The pertinent proton receptors have not been identified and may be ion channels. CRCs are probably neurons but may also include acid-sensitive glia and vascular cells that communicate with neurons via paracrine mechanisms. Retrotrapezoid nucleus (RTN) neurons are the most completely characterized CRCs. Their high sensitivity to CO(2) in vivo presumably relies on their intrinsic acid sensitivity, excitatory inputs from the carotid bodies and brain regions such as raphe and hypothalamus, and facilitating influences from neighboring astrocytes. RTN neurons are necessary for the respiratory network to respond to CO(2) during the perinatal period and under anesthesia. In conscious adults, RTN neurons contribute to an unknown degree to the pH-dependent regulation of breathing rate, inspiratory, and expiratory activity. The abnormal prenatal development of RTN neurons probably contributes to the congenital central hypoventilation syndrome. Other CRCs presumably exist, but the supportive evidence is less complete. The proposed locations of these CRCs are the medullary raphe, the nucleus tractus solitarius, the ventrolateral medulla, the fastigial nucleus, and the hypothalamus. Several wake-promoting systems (serotonergic and catecholaminergic neurons, orexinergic neurons) are also putative CRCs. Their contribution to central respiratory chemoreception may be behavior dependent or vary according to the state of vigilance.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

Retrotrapezoid nucleus, respiratory chemosensitivity and breathing automaticity

Breathing automaticity and CO(2) regulation are inseparable neural processes. The retrotrapezoid nucleus (RTN), a group of glutamatergic neurons that express the transcription factor Phox2b, may be a crucial nodal point through which breathing automaticity is regulated to maintain CO(2) constant. This review updates the analysis presented in prior publications. Additional evidence that RTN neurons have central respiratory chemoreceptor properties is presented, but this is only one of many factors that determine their activity. The RTN is also regulated by powerful inputs from the carotid bodies and, at least in the adult, by many other synaptic inputs. We also analyze how RTN neurons may control the activity of the downstream central respiratory pattern generator. Specifically, we review the evidence which suggests that RTN neurons (a) innervate the entire ventral respiratory column and (b) control both inspiration and expiration. Finally, we argue that the RTN neurons are the adult form of the parafacial respiratory group in neonate rats.

ATP release from non-excitable cells

All cells release nucleotides and are in one way or another involved in local autocrine and paracrine regulation of organ function via stimulation of purinergic receptors. Significant technical advances have been made in recent years to quantify more precisely resting and stimulated adenosine triphosphate (ATP) concentrations in close proximity to the plasma membrane. These technical advances are reviewed here. However, the mechanisms by which cells release ATP continue to be enigmatic. The current state of knowledge on different suggested mechanisms is also reviewed. Current evidence suggests that two separate regulated modes of ATP release co-exist in non-excitable cells: (1) a conductive pore which in several systems has been found to be the channel pannexin 1 and (2) vesicular release. Modes of stimulation of ATP release are reviewed and indicate that both subtle mechanical stimulation and agonist-triggered release play pivotal roles. The mechano-sensor for ATP release is not yet defined.

ATP facilitates glutamatergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

Recent work has shown that adenosine 5'-triphosphate (ATP) plays an important role in modulating the activity of parasympathetic cardiac vagal neurons that dominate the neural control of heart rate. This study examined the mechanisms by which activation of ATP receptors modulates excitatory neurotransmission to cardiac vagal neurons. Glutamatergic activity to cardiac vagal neurons was isolated and examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. ATP (100 microM) evoked increases in the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in cardiac vagal neurons which were blocked by the broad P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 100 microM). Application of the selective P2X receptor agonist, alpha, beta-methylene ATP (100 microM), also increased glutamatergic mEPSCs neurotransmission to cardiac vagal neurons indicating P2X receptors enhance glutamatergic release to cardiac vagal neurons. The evoked increase in glutamatergic mEPSC was unaltered by the voltage-gated calcium channel blocker cadmium, and was abolished by the selective P2X receptor antagonist 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate, TNP-ATP (100 microM). This work demonstrates that the ATP evoked facilitation of excitatory neurotransmission to cardiac vagal neurons is dependent upon activation of P2X receptors on glutamatergic presynaptic terminals.

IL-1beta inhibits IK and increases [Ca2+]i in the carotid body glomus cells and increases carotid sinus nerve firings in the rat

Increasing evidence indicates that there exists a reciprocal communication between the immune system and the brain. Interleukin 1beta (IL-1beta), a proinflammatory cytokine produced during immune challenge, is believed to be one of the mediators of immune-to-brain communication, but how it gets into the brain is unknown because of its large molecular weight and difficulty in crossing the blood-brain barrier. Our previous work has demonstrated that IL-1 receptor type I is strongly expressed in the glomus cells of rat carotid body (CB), a well characterized polymodal chemoreceptive organ which serves not only for the detection of hypoxia, hypercapnia and acidity, but also for low temperature and blood glucose. The present study was designed to test whether IL-1beta could stimulate the CB glomus cells and alter the discharge properties in the carotid sinus nerve, the afferent nerve innervating the organ. The results from whole-cell patch-clamp recordings and calcium imaging showed that extracellular application of IL-1beta significantly decreased the outward potassium current and triggered a transient rise in [Ca(2+)](i) in the cultured glomus cells of rat CB. Furthermore, by using extracellular recordings and pharmacological intervention, it was found that IL-1beta stimulation of the CB in the anaesthetized rat in vivo significantly increased the discharge rate in the carotid sinus nerve, most probably mediated by ATP release. This experiment provides evidence that the CB responds to cytokine stimulation and proposes the possibility that the CB might play a role in immune-to-brain communication.

Involvement of L-glutamate and ATP in the neurotransmission of the sympathoexcitatory component of the chemoreflex in the commissural nucleus tractus solitarii of awake rats and in the working heart-brainstem preparation

Peripheral chemoreflex activation with potassium cyanide (KCN) in awake rats or in the working heart-brainstem preparation (WHBP) produces: (a) a sympathoexcitatory/pressor response; (b) bradycardia; and (c) an increase in the frequency of breathing. Our main aim was to evaluate neurotransmitters involved in mediating the sympathoexcitatory component of the chemoreflex within the nucleus tractus solitarii (NTS). In previous studies in conscious rats, the reflex bradycardia, but not the pressor response, was reduced by antagonism of either ionotropic glutamate or purinergic P2 receptors within the NTS. In the present study we evaluated a possible dual role of both P2 and NMDA receptors in the NTS for processing the sympathoexcitatory component (pressor response) of the chemoreflex in awake rats as well as in the WHBP. Simultaneous blockade of ionotropic glutamate receptors and P2 receptors by sequential microinjections of kynurenic acid (KYN, 2 nmol (50 nl)(-1)) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonate (PPADS, 0.25 nmol (50 nl)(-1)) into the commissural NTS in awake rats produced a significant reduction in both the pressor (+38+/-3 versus +8+/-3 mmHg) and bradycardic responses (-172+/-18 versus -16+/-13 beats min(-1); n=13), but no significant changes in the tachypnoea measured using plethysmography (270+/-30 versus 240+/-21 cycles min(-1), n=7) following chemoreflex activation in awake rats. Control microinjections of saline produced no significant changes in these reflex responses. In WHBP, microinjection of KYN (2 nmol (20 nl)(-1)) and PPADS (1.6 nmol (20 nl)(-1)) into the commissural NTS attenuated significantly both the increase in thoracic sympathetic activity (+52+/-2% versus +17+/-1%) and the bradycardic response (-151+/-17 versus -21+/-3 beats min(-1)) but produced no significant changes in the increase of the frequency of phrenic nerve discharge (+0.24+/-0.02 versus +0.20+/-0.02 Hz). The data indicate that combined microinjections of PPADS and KYN into the commissural NTS in both awake rats and the WHBP are required to produce a significant reduction in the sympathoexcitatory response (pressor response) to peripheral chemoreflex activation. We conclude that glutamatergic and purinergic mechanisms are part of the complex neurotransmission system of the sympathoexcitatory component of the chemoreflex at the level of the commissural NTS.

Is ATP a suitable co-transmitter in carotid body arterial chemoreceptors?

A review is presented on carotid body ATP content, effects and release, receptors involved and results of their block by purinergic antagonists, and the possibility of cholinergic-purinergic co-transmission in the carotid body. Glomus cells release ACh and ATP upon physiological stimulation. Both agents and their agonists have chemo-excitatory actions and their combined effects disappear upon blocking n-ACh and P2X receptors. Both ACh and ATP also are capable of exciting the somata of chemosensory neurons of petrosal ganglia. Although a combined cholinergic-purinergic block suppresses the chemosensory activity in neurons co-cultured with glomus cells and some carotid body preparations in vitro, basal chemosensory activity and chemosensory responses to hypoxic stimuli persist in cat carotid body preparations in situ and in vitro. Therefore, ATP is an effective excitatory agent for carotid body chemosensory activity, although less potent than ACh; their joint participation may contribute to -- but does not entirely explain -- the transfer of chemoreceptor excitation from glomus cells to sensory endings in carotid body.

Lamotrigine and phenytoin, but not amiodarone, impair peripheral chemoreceptor responses to hypoxia

Amiodarone, lamotrigine, and phenytoin, common antiarrhythmic and antiepileptic drugs, inhibit a persistent sodium current in neurons (I(NaP)). Previous results from our laboratory suggested that I(NaP) is critical for functionality of peripheral chemoreceptors. In this study, we determined the effects of therapeutic levels of amiodarone, lamotrigine, and phenytoin on peripheral chemoreceptor and ventilatory responses to hypoxia. Action potentials (APs) of single chemoreceptor afferents were recorded using suction electrodes advanced into the petrosal ganglion of an in vitro rat peripheral chemoreceptor complex. AP frequency (at Po(2) approximately 150 Torr and Po(2) approximately 90 Torr), conduction time, duration, and amplitude were measured before and during perfusion with therapeutic dosages of the drug or vehicle. Hypoxia-induced catecholamine secretion within the carotid body was measured using amperometry. With the use of whole body plethysmography, respiration was measured in unanesthesized rats while breathing room air, 12% O(2), and 5% CO(2), before and after intraperitoneal administration of amiodarone, lamotrigine, phenytoin, or vehicle. Lamotrigine (10 microM) and phenytoin (5 microM), but not amiodarone (5 microM), decreased chemoreceptor AP frequency without affecting other AP parameters or magnitude of catecholamine secretion. Similarly, lamotrigine (5 mg/kg) and phenytoin (10 mg/kg) blunted the hypoxic but not the hypercapnic ventilatory response. In contrast, amiodarone (2.5 mg/kg) did not alter the ventilatory response to hypoxia or hypercapnia. We conclude that lamotrigine and phenytoin at therapeutic levels impair peripheral chemoreceptor function and ventilatory response to acute hypoxia. These are consistent with I(NaP) serving an important function in AP generation and may be clinically important in the care of patients using these drugs.

Carotid body chemosensory activity and ventilatory chemoreflexes in cats persist after combined cholinergic-purinergic block

Acetylcholine (ACh) and ATP have been proposed as excitatory co-transmitters operating at synapses between glomus cells and sensory nerve endings of the carotid body (CB). To test such hypothesis, we performed experiments on cats under pentobarbitone anesthesia and breathing spontaneously. Cholinergic and purinergic agonists and antagonists were given into one common carotid artery. Chemoreflex ventilatory changes initiated from the ipsilateral CB or chemosensory activity from the ipsilateral carotid nerve were recorded. Agonists ACh, nicotine, epibatidine, ATP, betagamma-methylene-ATP and gammaS-ATP induced transient chemoreflex enhancements of ventilation or increased chemosensory activity. When given in combination, mecamylamine and suramin suppressed both nicotine- and ATP-induced ventilatory chemoreflexes or chemosensory responses. However, neither chemoreflex hyperventilation induced by brief hypoxic exposures or steady-state hypoxic levels, nor chemosensory excitation elicited by these maneuvers were eliminated. Asphyxia-induced chemosensory excitation was not reduced by combined blockade of ACh and ATP receptors. Furthermore, ventilatory or chemosensory depression evoked by 100% O2 tests was unmodified, thus evidencing that basal chemosensory drive in normoxia was not suppressed by combined cholinergic-purinergic blockade. Therefore, although ACh and ATP may participate in chemoexcitation of the CB, their involvement fails to explain the origin of chemosensory discharges from synaptic transmission between glomus cells and chemosensory nerve endings of the CB.

Oxygen sensing in the body

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

An important functional role of persistent Na+ current in carotid body hypoxia transduction

Systemic hypoxia in mammals is sensed and transduced by the carotid body into increased action potential (AP) frequency on the sinus nerve, resulting in increased ventilation. The mechanism of hypoxia transduction is not resolved, but previous work suggested that fast Na(+) channels play an important role in determining the rate and timing of APs (Donnelly, DF, Panisello JM, and Boggs D. J Physiol. 511: 301-311, 1998). We speculated that Na(+) channel activity between APs, termed persistent Na(+) current (I(NaP)), is responsible for AP generation that and riluzole and phenytoin, which inhibit this current, would impair organ function. Using whole cell patch clamp recording of intact petrosal neurons with projections to the carotid body, we demonstrated that I(NaP) is present in chemoreceptor afferent neurons and is inhibited by riluzole. Furthermore, discharge frequencies of single-unit, chemoreceptor activity, in vitro, during normoxia (Po(2) 150 Torr) and during acute hypoxia (Po(2) 90 Torr) were significantly reduced by riluzole concentrations at or above 5 microM, and by phenytoin at 100 microM, without significant affect on nerve conduction time, AP magnitude (inferred from extracellular field), and AP duration. The effect of both drugs appeared solely postsynaptic because hypoxia-induced catecholamine release in the carotid body was not altered by either drug. The respiratory response of unanesthetized, unrestrained 2-wk-old rats to acute hypoxia (12% inspired O(2) fraction), which was measured with whole body plethysmography, was significantly reduced after treatment with riluzole (2 mg/kg ip) and phenytoin (20 mg/kg ip). We conclude that I(NaP) is present in chemoreceptor afferent neurons and serves an important role in peripheral chemoreceptor function and, hence, in the ventilatory response to hypoxia.

The E-NTPDase family of ectonucleotidases: Structure function relationships and pathophysiological significance

Ectonucleotidases are ectoenzymes that hydrolyze extracellular nucleotides to the respective nucleosides. Within the past decade, ectonucleotidases belonging to several enzyme families have been discovered, cloned and characterized. In this article, we specifically address the cell surface-located members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase/CD39) family (NTPDase1,2,3, and 8). The molecular identification of individual NTPDase subtypes, genetic engineering, mutational analyses, and the generation of subtype-specific antibodies have resulted in considerable insights into enzyme structure and function. These advances also allow definition of physiological and patho-physiological implications of NTPDases in a considerable variety of tissues. Biological actions of NTPDases are a consequence (at least in part) of the regulated phosphohydrolytic activity on extracellular nucleotides and consequent effects on P2-receptor signaling. It further appears that the spatial and temporal expression of NTPDases by various cell types within the vasculature, the nervous tissues and other tissues impacts on several patho-physiological processes. Examples include acute effects on cellular metabolism, adhesion, activation and migration with other protracted impacts upon developmental responses, inclusive of cellular proliferation, differentiation and apoptosis, as seen with atherosclerosis, degenerative neurological diseases and immune rejection of transplanted organs and cells. Future clinical applications are expected to involve the development of new therapeutic strategies for transplantation and various inflammatory cardiovascular, gastrointestinal and neurological diseases.

Enhanced c-Fos expression in the rostral ventral respiratory complex and rostral parapyramidal region by inhibition of the Na+/H+ exchanger type 3

Previous studies have shown that selective inhibition of Na+/H+ exchanger type 3 (NHE3) induces intracellular acidification and activates CO2/H+-sensitive medullary neurons, mimicking the responses evoked by hypercapnic stimuli. In addition, NHE3 blockers administration decreases the duration of apnoea induced by laryngeal stimulation, presumably by means of central chemoreceptor activation. To test the hypothesis that the central chemoreceptor network may be affected by NHE3 inhibition, brainstem c-Fos immunoreactive cell counting was performed after systemic administration of the NHE3 blocker AVE1599 (Aventis Pharma Deutschland GmbH) (2 mg/kg). The rostro-caudal quantitative c-Fos analysis showed a significant increase in the number of c-Fos positive cells in the rostral part of the ventral respiratory complex (VRC) as well as in the rostral part of the parapyramidal (Ppy) region. The VRC activated region (-4.2 to -3.2mm interaural) included the pre-Bötzinger complex, the rostral ventral respiratory group and the rostral ventrolateral medulla, all of them involved in cardiorespiratory control. The activated Ppy region corresponded with the rostral chemosensitive area, which elicits the strongest ventilatory response upon ventral medullary surface stimulation with H+/CO2. Most cells activated in Ppy after NHE3 inhibition were serotonergic. Hence, systemic application of NHE3 blockers may induce central chemoreceptors activation and an increase in the respiratory network activity in a similar way to known physiological stimuli such as hypercapnia. On the other hand, selective NHE3 blockers could be excellent tools for treatment of pathological states where central chemoreceptor function is diminished or absent, such as central hypoventilation syndrome or sudden infant death syndrome.

Hair Cells – Beyond the Transducer

OVERVIEW

This review considers the "tween twixt and twain" of hair cell physiology, specifically the signaling elements and membrane conductances which underpin forward and reverse transduction at the input stage of hair cell function and neurotransmitter release at the output stage. Other sections of this review series outline the advances which have been made in understanding the molecular physiology of mechanoelectrical transduction and outer hair cell electromotility. Here we outline the contributions of a considerable array of ion channels and receptor signaling pathways that define the biophysical status of the sensory hair cells, contributing to hair cell development and subsequently defining the operational condition of the hair cells across the broad dynamic range of physiological function.

Effect of adenosine triphosphate on phosphate uptake in renal proximal tubule cells: involvement of PKC and p38 MAPK

ATP has been known to act as an extracellular signal and to be involved in various functions of kidney. Renal proximal tubular reabsorption of phosphate (Pi) contributes to the maintenance of phosphate homeostasis, which is regulated by Na+/Pi cotransporter. However, the effects of ATP on Na+/Pi cotransporters were not elucidated in proximal tubule cells (PTCs). Thus, the effects of ATP on Na+/Pi cotransporter and its related signal pathways are examined in the primary cultured renal PTCs. In the present study, ATP inhibited Pi uptake in a time (> 1 h) and dose (>10(-6)M) dependent manner. ATP-induced inhibition of Pi uptake was correlated with the decrease of type II Na+/Pi cotransporter mRNA. ATP-induced inhibition of Pi uptake may be mediated by P2Y receptor activation, since suramin (non-specific P2 receptor antagonist) and RB-2 (P2Y receptor antagonist) blocked it. ATP-induced inhibition of Pi uptake was blocked by neomycin, U73122 (phospholipase C (PLC) inhibitors), bisindolylmaleimide I, H-7, and staurosporine (protein kinase C (PKC) inhibitors), suggesting the role of PLC/PKC pathway. ATP also increased inositol phosphates (IPs) formation and induced PKC translocation from cytosolic fraction to membrane fraction. In addition, ATP-induced inhibition of Pi uptake was blocked by SB 203580 [a p38 mitogen activated protein kinase (MAPK) inhibitor], but not by PD 98059 (a p44/42 MAPK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK, which was not blocked by PKC inhibitor. In conclusion, ATP inhibited Pi uptake via PLC/PKC as well as p38 MAPK in renal PTCs.

Assessment of the maturity-related brainstem functions reveals the heterogeneous phenotypes and facilitates clinical management of Rett syndrome

We have investigated whether brainstem assessment using the Neuro Scope could be used for objective and quantitative monitoring of early development and later progress in Rett syndrome. Brainstem features can be seen on bedside examination of Rett patients and are included in the International Scoring system. The following cardiovascular vital signs were recorded simultaneously in real-time: cardiac vagal tone (CVT), cardiac sensitivity to baroreflex (CSB), heart rate (HR), and mean arterial blood pressure (MAP) and respiratory vital signs: breathing rate and pattern, transcutaneous partial pressures of oxygen (pO(2)) and carbon dioxide (pCO(2)). We assessed the occipito-frontal head circumference (OFHC), height and body mass index (BMI). Results are from 72 patients with classical Rett syndrome studied at the Swedish National Rett Centre. Three cardiorespiratory phenotypes, designated Forceful, Feeble and Apneustic breathers were present in similar proportions in the Rett population but early development measured by OFHC and BMI differed. Height was not affected. Baseline levels of CVT and CSB also differed within the phenotypes indicating differences in parasympathetic activities. However, parasympathetic activity in the whole population was similar to that previously reported in Rett. Baseline levels of MAP and HR were similar across the phenotypes, consistent with previous reports of little effect of Rett disorder on baseline sympathetic tone. Adverse responses to opiate analgesics and hypocapnoeic attacks were unique to specific phenotypes. We recommend early characterisation of these phenotypes in the management of Rett syndrome. We conclude that classical Rett syndrome consists of heterogeneous clinical phenotypes with distinct cardiorespiratory states. Brainstem functions can be used to identify these and to monitor development and treatment, thereby facilitating clinical management.

Direct Excitation of Hypocretin/Orexin Cells by Extracellular ATP at P2X Receptors

Hypocretin/orexin (hcrt) neurons play an important role in hypothalamic arousal and energy homeostasis. ATP may be released by neurons or glia or by pathological conditions. Here we studied the effect of extracellular ATP on hypocretin cells using whole cell patch-clamp recording in hypothalamic slices of transgenic mice expressing green fluorescent protein (GFP) exclusively in hcrt-producing cells. Local application of ATP induced a dose-dependent increase in spike frequency. In the presence of TTX, ATP (100 μM) depolarized the cells by 7.8 ± 1.2 mV. In voltage clamp under blockade of synaptic activity with the GABAA receptor antagonist bicuculline, and ionotropic glutamate receptor antagonists dl-2-amino-5-phosphonopentanoic acid (AP-5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), ATP (100 μM) evoked an 18 pA inward current. The inward current was blocked by extracellular choline substitution for Na+, had a reversal potential of −27 mV, and was not affected by nominally Ca2+-free external buffer, suggesting that ATP activated a nonselective cation current. All excitatory effects of ATP showed rapid attenuation. ATP-induced excitatory actions were mimicked by nonhydrolyzable ATP-γ-S but not by α,β-MeATP and inhibited by the purinoceptor antagonists suramin and pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt (PPADS). The current was potentiated by a decrease in bath pH, suggesting P2X2 subunit involvement. Frequency and amplitude of spontaneous and miniature synaptic events were not altered by ATP. Suramin, but not PPADS, caused a small suppression of evoked excitatory synaptic potentials. Together, these results show a depolarizing response to extracellular ATP that would lead to an increased activity of the hypocretin arousal system.

ATP stimulates Na+-glucose cotransporter activity via cAMP and p38 MAPK in renal proximal tubule cells

Extracellular ATP plays an important role in the regulation of renal function. However, the effect of ATP on the Na(+)-glucose cotransporters (SGLTs) has not been elucidated in proximal tubule cells (PTCs). Therefore, this study was performed to examine the action of ATP on SGLTs and their related signal pathways in primary cultured rabbit renal PTCs. ATP increased [(14)C]-alpha-methyl-d-glucopyranoside (alpha-MG) uptake in a time-dependent (>1 h) and dose-dependent (>10(-6) M) manner. ATP stimulated alpha-MG uptake by increasing in V(max) without affecting K(m). ATP-induced increase of alpha-MG uptake was correlated with the increase in both SGLT1 and SGLT2 protein expression levels. ATP-induced stimulation of alpha-MG uptake was blocked by suramin (nonspecific P2 receptor antagonist), RB-2 (P2Y receptor antagonist), and MRS-2179 (P2Y(1) receptor antagonist), suggesting a role for the P2Y receptor. ATP-induced stimulation of alpha-MG uptake was blocked by pertussis toxin (PTX, a G(i) protein inhibitor), SQ-22536 (an adenylate cyclase inhibitor), and PKA inhibitor amide 14-22 (PKI). ATP also increased cAMP formation, which was blocked by PTX and RB-2. However, pretreatment of adenosine deaminase did not block ATP-induced cAMP formation. In addition, ATP-induced stimulation of alpha-MG uptake was blocked by SB-203580 (p38 MAPK inhibitor), but not by PD-98059 (p44/42 MAPK inhibitor) or SP-600125 (JNK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK. In conclusion, ATP increases alpha-MG uptake via cAMP and p38 MAPK in renal PTCs.

Release of ATP in the ventral medulla during hypoxia in rats: role in hypoxic ventilatory response

P2X2 receptor subunits of the ATP-gated ion channels are expressed by physiologically identified respiratory neurons in the ventral respiratory column, implicating ATP in the control of respiratory activity. We now show that, during hypoxia, release of ATP in the ventrolateral medulla (VLM) plays an important role in the hypoxic ventilatory response in rats. By measuring ATP release in real time at the ventral surface of the medulla with novel amperometric biosensors, we found that hypoxia (10% O2; 5 min) induced a marked increase in the concentration of ATP (approximately 3 microm). This ATP release occurred after the initiation of enhanced respiratory activity but coincided with the later hypoxia-induced slowing of the respiratory rhythm. ATP was also released at the ventral surface of the medulla during hypoxia in peripherally chemodenervated animals (vagi, aortic, and carotid sinus nerve sectioned). By using horizontal slices of the rat medulla, we found that, during hypoxia, ATP is produced throughout the VLM in the locations corresponding to the ventral respiratory column. Blockade of ATP receptors in the VLM (microinjection of P2 receptor antagonist pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid; 100 mum) augmented the hypoxia-induced secondary slowing of the respiratory rhythm. Our findings suggest that ATP released within the ventral respiratory column is involved in maintenance of the respiratory activity in conditions when hypoxia-induced slowing of respiration occurs. These data illustrate a new functional role for ATP-mediated purinergic signaling in the medullary mechanisms controlling respiratory activity.

CO2 central chemosensitivity: why are there so many sensing molecules?

CO2 central chemoreceptors (CCRs) play a critical role in respiratory and cardiovascular controls. Although the primary sensory cells and their neuronal networks remain elusive, recent studies have begun to shed insight into the molecular mechanisms of several pH sensitive proteins. These putative CO2/pH-sensing molecules are expressed in the brainstem, detect P(CO2) at physiological levels, and couple the P(CO2) to membrane excitability. Functional analysis suggests that multiple CO2/pH-sensing molecules are needed to achieve high sensitivity and broad bandwidth of the CCRs. In contrast to the diversity of pH sensitive molecules, molecular mechanisms for CO2 sensing are rather general. The sensing molecules detect pH changes rather than molecular CO2. One or a few titratable amino acid residues in these proteins are usually involved. Protonation of these residues may lead to a change in protein conformation that is coupled to a change in channel activity. Depending on the location of the protonation sites, a membrane protein can detect extra- and/or intracellular pH.

Respiratory control by ventral surface chemoreceptor neurons in rats

A long-standing theory posits that central chemoreception, the CNS mechanism for CO(2) detection and regulation of breathing, involves neurons located at the ventral surface of the medulla oblongata (VMS). Using in vivo and in vitro electrophysiological recordings, we identify VMS neurons within the rat retrotrapezoid nucleus (RTN) that have characteristics befitting these elusive chemoreceptors. These glutamatergic neurons are vigorously activated by CO(2) in vivo, whereas serotonergic neurons are not. Their CO(2) sensitivity is unaffected by pharmacological blockade of the respiratory pattern generator and persists without carotid body input. RTN CO(2)-sensitive neurons have extensive dendrites along the VMS and they innervate key pontomedullary respiratory centers. In brainstem slices, a subset of RTN neurons with markedly similar morphology is robustly activated by acidification and CO(2). Their pH sensitivity is intrinsic and involves a background K(+) current. In short, the CO(2)-sensitive neurons of the RTN are good candidates for the long sought-after VMS chemoreceptors.